ENGLISH ABSTRACT: In vitro and in situ methods using the in sacco technique have a wide application in ruminant
nutrition as they allow the degradability and quality of forages and ruminant diets to be
determined quicker and at a lower cost than in vivo methods. These trials make use of artificial
fibre bags, made of polyester (dacron) or nylon which are available in variable pore sizes.
Results from such degradability trials are of great value to feed formulation programs such as
AMTS.cattle and CPM Dairy and the more accurate the results are obtained from such trials the
more accurate feed formulation models are enabling the ruminant nutritionist to formulate the
best diet possible to reach the genetic potential of ruminants. The accepted method for in sacco
trials (NRC, 2001) requires that the feed samples are ground through a 2 mm screen. This
usually results in a variety of particle sizes, including a significant amount of extremely fine
particles. Research has, however, shown that these fine particles can potentially be washed out
of the dacron bags that are used in in sacco degradability trials. This would result in an overestimation
of the soluble and rapidly fermentable nutrient fractions. The objective of this study
was to determine the effect of fine particle removal from ground forage samples on the chemical
composition and in vitro dry matter (DM) and neutral detergent fibre (NDF) degradability of
forages. Lucerne hay, oat hay and wheat straw samples were sourced from seven different
locations in the Western Cape. Samples were milled through a 2 mm screen and then sieved
through either 150 μm, 125 μm or 106 μm. All fractions were analysed for DM, crude protein
(CP), NDF, fat and ash. Based on the NDF content of the original samples, four samples from
each forage type were selected for in vitro trials to determine DM and NDF disappearance over
time. Samples were incubated for 0, 6, 24 and 48 hours in an ANKOM Daisy II incubator.
Significant variation occurred within forage types in terms of chemical composition. Fine particle removal had no effect on the NDF content of lucerne hay and wheat straw, but sieved oat hay
fractions had a higher NDF content than the un-sieved samples. The NDF content was on
average 635.9 for the sieved OH fractions, whereas the NDF content of the un-sieved samples
was 606.8. The CP content of sieved oat hay (61.4 on average) and wheat straw fractions (47.7
on average) were lower than the un-sieved fractions (65.7 for OH and 55.4 for WS), whereas for
lucerne hay, sieving had no effect on CP content.
Dry matter and NDF disappearances were significantly higher for the un-sieved samples than
for the sieved fractions for all three forage types at all incubation times, which indicates an overestimation
of the soluble and readily digestible forage fractions. Compared to sieved samples,
DMD values at 0 hours (washing only) of the un-sieved samples were, on average, 13.8%
higher for lucerne hay, 27.3% for oat hay and 44.7% for wheat straw. At 48 h, the overestimation
of lucerne DMD for the un-sieved samples was between 4.0% (compared to 106 μm
sieve) and 7.3% (compared to 150 μm sieve). This over-estimation in the un-sieved samples
was carried over to all four time points. No significant differences between the fractions (150,
125 and 106) were found within a forage type at all incubation times. The estimated degradation
rates and the predicted digested proportions were also significantly higher for the un-sieved
fractions compared to the sieved fractions. It was concluded that fine particle removal from
forage samples would result in more accurate estimations of in sacco nutrient degradability.